The term fluoroscopy refers to the use of x-ray imaging techniques for real-time visualization of internal anatomy and function for diagnostic and therapeutic purposes. Physiologic functions such as peristalsis and flow, and real-time image feedback for placement of devices, such as catheters or intravascular stents are typical examples of fluoroscopic imaging. However, fluoroscopy at 30 video frames/second alone is of limited use without the capability of switching to a high detail mode. This mode may be activated by a command to produce a spot image ‘snapshot’ and in many applications acquisition of rapid sequences of spot images or high detail images at a higher radiation dose are essential. In this mode, the system operates in a rapid sequence radiographic mode, where the exposure per frame at the entrance of the fluoroscopic imaging system is increased from the typical 1 to 3-μR per video frame (fluoroscopic mode) to about 300-μR per frame (radiographic mode). In fluoroscopy, the ability to change the spatial resolution during the examination enables physicians to focus on a smaller area and visualize with greater detail. Although the traditional role of fluoroscopy provides enough justification of the importance of maintaining and improving image quality at a reduced radiation dose, in the past few years the role of fluoroscopy has greatly expanded to cover many more diagnostic and therapeutic applications. More interventional fluoroscopic procedures are performed today in younger patients as an alternative to surgery.
In spite of recent developments in non-invasive procedures such as, magnetic resonance imaging, ultrasound and computed tomography, x-ray fluoroscopy remains the “gold-standard” for procedures such as diagnostic percutaneous coronary angiography, angioplasty, stent placement, pacemaker placement, electrophysiology, and peripheral vascular procedures. The success of these procedures is making a major impact not only in the survival rate of patients from cardiovascular disease, but also on the overall quality of life. As these procedures become more effective, younger patients are increasingly becoming candidates for such procedures. It is now common for young patients to undergo cardiac radiofrequency ablation procedures. There are also clinical situations such as the evaluation of coronary artery patency following thrombolysis or in the operating room to assess graft patency where compact bedside angiographic equipment can be extremely useful.
Video pick-up tube-based image intensifiers for fluoroscopy was invented in about 1940 and has been in use since 1948 when Coltman built the first practical image intensifier. Now, image intensifiers are a standard and essential component of fluoroscopic systems. Although several aspects of this technology have evolved over the years, the basic approach of detection remains the same. Image intensifier technology with video tube-based cameras and more recently charge-coupled devices (CCDs) have made a major impact in the field of x-ray fluoroscopy. In spite of the technical improvements, this technology suffers from several inherent limitations. Veiling glare and contrast loss is one of the more typical problems inherent in the electro-optic design of the image intensifier. After conversion of the light from the scintillator to the photocathode, electrons are accelerated in a field potential of about 30 kV. During this stage, a fraction of the electrons undergo scatter within the tube. At the output stage, after conversion from electrons to photons, the light scatters within the optical elements of the output. S-type distortion is also a well-known phenomenon, which makes imaging of a straight object to appear as having an S-shape due to the influence of the earth's magnetic field on the trajectories of electrons within the image-intensifier tube. Shielding of image intensifiers with “mu-metal” is essential but in many cases a significant amount of S-type distortion is still present. This distortion is not only bothersome during treatment procedures requiring high spatial accuracy, but also changes spatially as the intensifier is moved, making it difficult to correct mathematically. Other types of distortion such as pincushion and barrel type distortions are caused by the inherent limitations of the electron focusing optics. Pincushion and barrel distortions are tolerable in many instances but they present a hindrance in the proper visualization of anatomy. Similar effects but for different physical reasons also arise from lens-based optical coupling. The glass input window typically has been the input window of image intensifiers (typically 1 to 3-mm thick), which absorbs useful x-rays and produces forward scatter, but has now been replaced with a thickness of 0.7 to 1.2-mm, of aluminum (Al). While this represents a significant improvement, the input window itself absorbs about 20 to 30% of the useful x-ray beam depending on the photon energy. The high vacuum of the intensifier requires a relatively thick metal window for maintaining mechanical integrity of the tube. In addition to this aluminum layer of the input window, x-rays must pass through another 0.5-mm thick aluminum layer, the scintillator substrate, before they reach the scintillator. In addition, the gain of image intensifiers is known to degrade with time due in part to out-gassing of components in the vacuum chamber and degradation of the photocathode. The image quality is noticeably lower after three years of operation and their useful lifetime, if good image quality is to be maintained, is about 3 to 5 years. Also, the relatively large size of image intensifiers may be problematic in biplanar installations. Even in simple fluoroscopic installations the camera tower frequently interferes with the overhead radiographic x-ray tube and other structures.
Image intensifier and electronic readout technology has evolved significantly over the years and the image quality of modern image intensifier with CCD readout is far superior to the earlier approaches. However, radiation exposure to patients during diagnostic and interventional cardiac procedures has increased as a result of the increased complexity of the angiographic procedures performed in current clinical practice. The rapid proliferation of these procedures has resulted in a small but alarming number of non-stochastic radiation effects on patients. These include epilation, erythema and tissue necrosis in a number of cases. Cardiac angiography produces one of the highest radiation exposures of any commonly used diagnostic x-ray procedure. Recently, chronic dermatitis has been reported after repeated therapeutic interventional procedures using prolonged fluoroscopic imaging. All these factors indicate not only the need for safe and good fluoroscopic habits but also the need for developing an alternate technology, which is capable of improving image quality at an even reduced radiation dose. Early attempts have focused on flat panel intensifiers, typically using microchannel plates or solid state detectors. In the past few years, several research groups have been working on developing new technologies and improving existing technology for fluoroscopic applications. While there may be applications where one type of technology is preferable than the other, at this time there still remains a need to provide a higher level of image quality at a minimum radiation dose.